Organocobalt carbonyl complexes

Jhe reaction mechanism of the various metal complexes clearly have much in common. The recently recognized insertion reaction appears to be a particularly good example of a reaction which is general among the metal compounds. In the following discussion I intend to point out the generality of the insertion reaction with examples from the literature and from our own wrork. The most complete series of substantiated insertion reactions involves the organocobalt carbonyl complexes, and these reactions will form the nucleus of the discussion. 

The treatment of organocobalt carbonyl complexes formed from acetylene and cobalt carbonyl with phosphines also gives organocobaltcarbonylphos-phine complexes, e.g., Co2(CO)4(C6H5C=CC6Hs)[P(C6H5)3]2 (312). 

Only a few other cobalt complexes of the type covered in this review (and therefore excluding, for example, the cobalt carbonyls) have been reported to act as catalysts for homogeneous hydrogenation. The complex Co(DMG)2 will catalyze the hydrogenation of benzil (PhCOCOPh) to benzoin (PhCHOHCOPh). When this reaction is carried out in the presence of quinine, the product shows optical activity. The degree of optical purity varies with the nature of the solvent and reaches a maximum of 61.5% in benzene. It was concluded that asymmetric synthesis occurred via the formation of an organocobalt complex in which quinine was coordinated in the trans position (133). Both Co(DMG)2 and cobalamin-cobalt(II) in methanol will catalyze the following reductive methylations ... 

Reduction to Alcohols. The organosilane-mediated reduction of ketones to secondary alcohols has been shown to occur under a wide variety of conditions. Only those reactions that are of high yield and of a more practical nature are mentioned here. As with aldehydes, ketones do not normally react spontaneously with organosilicon hydrides to form alcohols. The exceptional behavior of some organocobalt cluster complex carbonyl compounds was noted previously. Introduction of acids or other electrophilic species that are capable of coordination with the carbonyl oxygen enables reduction to occur by transfer of silyl hydride to the polarized carbonyl carbon (Eq. 2). This permits facile, chemoselective reduction of many ketones to alcohols. 

Both organocobalt(I) and (II) carbonyl complexes can be synthesized from te-trahydrofuran (THF) solutions of bis(pentafluorophenyl)cobalt(II), and this is an... 

Besides organocobalt complexes, organostibine has also been found to be able to mediate controlled/ diving polymerization of many vinyl monomers. " For example, Yamago et al reported that at 60 °C the polymerization of VAc mediated by a-dimethylstibanyl ester reached 92% conversion in 5 h and produced narrowly dispersed PVAc (PDl = 1.26), but no detailed reaction kinetics was provided in their study.Kamigaito et al. very recently found that a manganese carbonyl complex [Mn2(CO)io] coupled with an alkyl iodide ( -I)... 

A reassessment of the experimental data concerning the catalytic hydrogenation of polyaromatic hydrocarbons by cobalt carbonyl under oxo conditions has resulted in the postulate that the reaction involves free radicals rather than organocobalt complexes. ... 

---